Interstellar Arrival: Slowing the Sail

byPaul GilsteronNovember 7, 2014

Some final thoughts on hybrid propulsion will wrap up this series on solar sails, which grew out of ideas I encountered in the new edition of the Matloff, Johnson and Vulpetti book Solar Sails: A Novel Approach to Interplanetary Travel (Copernicus, 2014). The chance to preview the book (publication is slated for later this month) took me in directions I hadn’t anticipated. Solar Sails offers a broad popular treatment of all the sail categories and their history, as you’d expect, but this time through I focused on its four technical chapters on sail theory that helped me review the details.

And because I kept running into the idea of multiple modes of propulsion, my thoughts on avoiding doctrinaire solutions continue to grow. In fact, I’d venture to say that probing into the possibilities of multimodal propulsion may offer a serious opportunity for insights. Centauri Dreams regular Alex Tolley came up with one of these yesterday, asking whether a sail mission to Jupiter space might deploy the planet’s huge magnetic field as an assist. Alex invokes Pekka Janhunen’s ideas about electric sails. Let me quote from the Solar Sails book on what Janhunen has in mind:

Similar to the magsail, this concept uses the solar wind for producing thrust. However, different from the magsail, this sail interacts with the solar plasma via a mesh of long and thin tethers kept at high positive voltage by means of an onboard electron gun. In its baseline configuration, the spacecraft spins and the tethers are tensioned by centrifugal acceleration. It should be possible to control each wire voltage singly, at least to within certain limits.

We get thrust out of this when protons from the solar wind, positively charged, are repelled by the positive voltage of the spacecraft’s tethers, while electrons are captured and ejected — otherwise, their growing numbers would neutralize the voltage in the tether mesh. But Alex also brings to mind Mason Peck’s interesting work at Cornell on miniaturized ‘Sprites,’ tiny chip-like spacecraft that could use the Lorentz force to accelerate in directions perpendicular to the magnetic field. Remember that Jupiter’s magnetic field is 18,000 times stronger than Earth’s, a useful resource if we can tap it even so far as to adjust the orbits of planetary probes.

Alex’s thoughts on the matter deserve to be quoted:

We often think of sail ships as clipper ships – i.e. using large surfaces to capture or direct the wind to move. But modern ships use screws. There have also been numerous wind turbine designs that offer advantages over canvas sails, even if they are not as aesthetic to the eye. (Clipper ships were possibly the most pleasing ship designs ever built). Might we be thinking too much in terms of sails that mimic the romance of traditional sails, rather than designs that might offer better performance, albeit with some aesthetic loss?

Interstellar Arrival

A sense of aesthetics produces pleasing designs but what looks best isn’t always what we need. Back in my flying days some of us used to talk about (and in a few cases actually fly) some of the great aircraft designs of the 1930s and later, and although I never got my hands on the controls of one, a great favorite was the Beech Staggerwing, a gorgeous design with a negative wing stagger, meaning that the lower wing is farther forward than the upper. Designs like this could be sleek and lovely because of the medium they worked in. But spacecraft don’t need wings and streamlined fuselages, and our Voyagers and Cassinis look nothing like the wilder designs of early science fiction because they don’t need to, never encountering a planetary atmosphere.

Image: The Beech Model 17 Staggerwing, first produced in 1932. Credit: Wikimedia Commons.

A beamed lasersail on its way to Alpha Centauri may be anything but a thing of beauty. Once the mission enters its cruise phase, the sail can be safely stowed, and one good use for it would be to shroud the payload to offer additional protection against radiation. We’re always trying to think of ways to get more value out of existing assets, which is what extended missions are all about. Or think about the Benfords’ JPL work that revealed desorption. No one with an eye for design would come up with painting a desorption layer on a sailcraft, but it’s conceivable that desorption, which is the release of CO2, hydrocarbons and hydrogen from within the manufactured sail as it heats up under the beam, could give an added kick to interplanetary sails being pushed by powerful microwave beams.

Mentioning Forward brings me back around to the ‘staged sail’ concept that he worked out for stopping at another star. The sail has three divisions, as shown in the diagram below, which is taken from his paper on a manned mission to Epsilon Eridani. ‘Staging’ the sail means losing first the outer ring, then the middle one, until only the inner ring is left. In sequence, the spacecraft slows down by using laser light beamed from our Solar System, reflected off the now separated outer sail as it approaches the star — the light is directed back at the two remaining sail segments with payload. Ingenious tinkering let Forward use the second sail detachment as the way the crew got home, with laser light boosting the much smaller inner sail by reflection from the middle segment.

Image: Robert Forward’s staged sail concept. What he calls a ‘paralens’ in the diagram is an enormous Fresnel lens in the outer Solar System, made of concentric rings of lightweight, transparent material with free space between the rings. Credit: Robert Forward.

Staged sails are hard to see as anything but a longshot — the success of the mission depends not only upon perfect execution of the staging process but, crucially, upon the laser beam from Earth being able to illuminate the sail segments effectively. Forward was fully aware of the possibilities here, and you can find discussion in places like his novel Rocheworld (Baen, 1990) about how politics on Earth might affect the use of the expensive beam. I for one wouldn’t want to put my life in the hands of a design like this, which depends so crucially upon decisions made far from the spacecraft.

Interestingly, like Mason Peck, Forward had some thoughts on how we might use the Lorentz force as well. Remember that a charged object moving through a magnetic field experiences this force at right angles to its direction of motion and the magnetic field itself. Out of this you get ‘thrustless turning,’ which both Forward and Philip Norem thought could be used for deceleration. Instead of staged sails, you get an electrostatically charged probe — think of Janhunen’s electric sail tethers — on a trajectory that goes well beyond the target star. The spacecraft’s interactions with the galactic magnetic field bend its trajectory so that it approaches the target from behind.

Once it’s inbound to the destination system, a laser beam from Earth can be turned upon it to slow it down for arrival. The idea is anything but aesthetic, just as the Janhunen sail would look like something closer to a porcupine than the silvery lozenge of an early SF starship. It’s also hampered by the fact that mission times, already measured in decades at minimum, are tripled with the use of this maneuver. I should mention that Solar Sails authors Gregory Matloff and Les Johnson have also explored the uses of electrodynamic tethers to supply power to an Alpha Centauri expedition, even if a Norem-style arrival seems too lengthy.

Creative thinking about these matters often springs from putting two or more solutions together to see what can happen. What I’ve always admired about the interstellar community is its ability to re-examine older concepts to look for interesting cross-pollination of ideas. As we move into the era of increasingly tiny components, it’s heartening to think how many designs will be affected by new nanotechnological possibilities. Mason Peck has talked about using Jupiter’s magnetic field to spew thousands of ‘Sprites’ out on interstellar trajectories. What else can we imagine as we look for extended uses of existing tech and ponder where they might lead us?

I think of a magsail as a magnetic parachute for decelerating. However, is it possible to reconfigure a sail to become a ramscoop instead, using the scoop’s drag to advantage here, as well as the fusion rocket’s thrust to decelerate? If so, then it might be worth examining the tradeoffs this hybrid system.

The engines needn’t be fusion rockets either, just solid state ion spray engines using the charged stellar wind as the propellant, making this suitable for small cubesat class payloads as well as larger craft.

I don’t think we need to a priori assume that resources in the target system cannot be utilized until the craft is nearly at rest, as is commonly envisioned. It might be possible to actively build elements of a deceleration device (e.g. electric sail) from scooped materials as the craft decelerates, thus increasing its performance at both ends of the journey, using the sparse material scooped up while decelerating from a small fractions of c (~1300 km/s for the 1000 years to alpha Centauri solar sail scenario).

‘I think of a magsail as a magnetic parachute for decelerating. However, is it possible to reconfigure a sail to become a ramscoop instead, using the scoop’s drag to advantage here, as well as the fusion rocket’s thrust to decelerate? If so, then it might be worth examining the tradeoffs this hybrid system.’

If you collect the fuel you have two problems, one you increase the mass of the craft to slow down by collecting it and second the ignition of the engine will clear the fuel from in front of the craft. Unless the scoop collects the material and bends it almost 180 degrees so that the deflected fuel acts as a greater sized scoop funnelling the material in to be turned around.

Perhaps we could send out an adjustable beam of material first by particle beam, the craft will then intercept the material later on in the flight and use it to slow down.

@Michael “If you collect the fuel you have two problems, one you increase the mass of the craft to slow down by collecting it”

Only if you collect it. If you burn it off as it is collected, I don’t think this should be a problem.

“second the ignition of the engine will clear the fuel from in front of the craft.”

I would have thought the reverse – the scoop will try to collect the exhaust, making it useless for thrust. It may be totally impractical, or even theoretically silly. But yes, there are many issues, which is why I asked the question. It seems an obvious approach and therefore may have been addressed in the past, especially when the Bussard ram jet idea was shown likely be be unworkable, resulting in more drag than engine thrust.

“The spacecraft’s interactions with the galactic magnetic field bend its trajectory so that it approaches the target from behind.”

Astronomers are only recently getting a better idea of some of symmetries of the galactic magnetic field. I’ll venture to open up the possibility, which would have been on the ragged fringe not long ago, to say that the properties of the galactic magnetic field, and the wide and regional co-existing currents that must be entwined with those fields, might well influence our choice of destination for a magnetic/electric hybrid probe.

Lorentz turning is a fascinating subject. As ProjectStudio mentions, the galactic field is not homogeneous at all. Navigating it by Lorentz turning might be much like navigating balloons (or sailships, for that matter) on Earth: Indirect and using complex maneuvers.

‘But spacecraft don’t need wings and streamlined fuselages, and our Voyagers and Cassinis look nothing like the wilder designs of early science fiction because they don’t need to, never encountering a planetary atmosphere.’

As we get faster and faster the particles that the craft will encounter will behave like an on coming wind, this will necessitate a streamlined craft of thin dimensions. Streamlined so that an impact will more likely be deflected to the side and thin to avoid collisions altogether. Very fast craft in the future may be designed more like a needle than a blunt nosed hammer.

Michael, as we get up into these speed ranges, we’ll also be needing serious shielding against what impacts do occur. Lots to work on there as we conceive of spacecraft moving at even modest percentages of lightspeed.

Beaming from close solar orbit is what is envisioned here, allowing us to extract the huge energies we’ll need to power up these futuristic laser arrays. This is what people like Forward were talking about, a space-based infrastructure that could sustain the power stations in space close to the Sun that were needed for the ambitious missions he was thinking about.

@Horatio – While there is carbon fuel in abundance on Titan, there is no free oxygen to burn it with. Sometime in the future, it may be that Titan’s hydrocarbons become the largest source of feed stocks for carbon structures and artifacts. Sail transport might make this viable economically, rather than brute force synthesis from carbonate rocks and water.

Alex Tolley, in principle you might be able to lift carbon out of Titan’s gravity well for 11% the energy cost of reducing CO2, but such a scheme would suffer multiple problems if we are ever to expect it as a vision of the future. I will try to list them.

1 We are assuming the oxygen is a waste product to be discarded. If we need O2 we would find a lower cost in obtaining it from (cometary) CO2 than water. The C would then come free.
2 If he have ANY H2 freely available, once more we get C for free from CO2 with water as the waste product.
3 Titan’s spin is so slow we could never build a space elevator there. We can however take its N2 for free if we can ever combine waverider technology with the slingshot effect. Thus huge amounts of N2 (for O’Neill habitats) might come from Titan, but only about 2% carbon should tag along.
4 There is oodles of C everywhere, except the inner solar system – organics being more common than rock. A very short space elevator on Haumea would just spew it out to wherever we wanted – to give but one example.

‘Michael, as we get up into these speed ranges, we’ll also be needing serious shielding against what impacts do occur. Lots to work on there as we conceive of spacecraft moving at even modest percentages of lightspeed.’

We currently have the tech to stop high energy atomic impacts very easily,
surprisingly a 300 micron aluminium/Mylar layer is enough to stop 4.5 MeV protons (10% c) or reduce the velocity of a significant number of them. A nice thing about a needle design is that a very high positive voltage can be produced at the tip which is strong enough to deflect positive charges away from the craft. Dust is also quite rare in space and not massive at all ~10^-17 kg.

@ Rob – And thus are the Saudi States of Titan stillborn. Someone should create a website collecting these sorts of calculations so that space resources can be categorized by availability and energy cost. Good to know that N2 is Titan’s most valuable resource when greening the solar system.

Penetration depth increases pretty rapidly with energy, and more rapidly with velocity. At 30% c you already will need an inch or so of aluminum, I think. Using charge (or magnetic fields) to deflect the ISM would be a good idea, in principle, if only the ISM were to cooperate and be charged. Alas, it isn’t, at least not in our galactic neighborhood. So, we’d have to efficiently ionize it ahead of the vehicle, somehow, which creates whole new levels of complexity and mass requirements. Better to just absorb it in a shield, I suppose.

At around 30% c the radiation that was ISM also starts to deposit large amounts of heat, which will require the shields not only to be thick, but also refractory and white hot in order to reradiate the imparted energy. This effect goes up with the third power of velocity, and becomes very, very limiting beyond 30% c. Tungsten or carbon may be the two materials best suited to the task, rather than aluminum.

If you work out the energy of hitting the ISM gas (1/cm^3) at 0.3 c it is quite low 0.154 W/cm^2, even an ISM dust particle ~10^-17 kg will do little. As for the neutral particles a strong electric field will have the effect of ionising them as they approach it and then perhaps a magnetic field could be used to deflect them. We must be careful with the type of materials used as at high enough energies neutron spalling becomes a problem.

With larger craft I thought about using a radiation channel all the way through the crafts stages, they don’t have to physically touch, from the reaction chamber exhaust. The high energy light radiation and neutrons would be reflected off the low angled walls of the channel and out the front (the channel entrance could be adjustably magnetically pinched to allow charged radiation through). The radiation would be sufficient to not only ionise neutral gases but also move the ionised ones as well using the gamma ray/neutron component.

0.154 W/cm^2 is brighter than sunshine, so it is not really low. And it rises very fast at higher velocities. I am not sure you can ionize the ISM with radiation. It would require incredibly dense radiation to be sure to hit most of the atoms.

Michael, I get a different result from yours: 0.3 c is 10^8 m/s, or 10^10 cm/s. That means 10^10 protons per second per cm^2. Each proton has a mass of 1.7*10^-27 kg, or an energy of 0.5*mv^2 ~ 8*10^-8 Joule. Power is incidence rate (10^10/s) times energy (~8*10^-8 J), coming to about 800 W/cm^2. This is 5000 times your result, and it implicates an equilibrium temperature of more than 7000 K, which is hotter than the surface of the sun and cannot be withstood by any materials.

I am not sure what is wrong here. May be you could double-check my calculation and point out the mistake?

When using a solid shield, assuming my above calculation is correct, the maximum velocity we could go is around 0.1 c. Beyond that, blocking shields are going to get too hot. An alternative to a fully blocking shield would be a thin foil, which could strip the electrons from the incoming hydrogen atoms without absorbing too much energy to radiate back into space. The remaining protons could then be diverted somehow by electromagnetic fields so they never impact any part of the ship. The foil would be subject to heavy erosion and would have to be constantly renewed, in my estimation. Whether the resulting consumable mass burden is prohibitive remains to be calculated.

Others have suggested droplet spray as a shield, which presumably is afflicted with a similar or worse consumable mass burden.

Michael and Eniac, I have noticed that estimates of the ISM density seem to vary greatly, but not by the orders of magnitude that would relsolve your disagreement.

Eniac, I note you continue to concentrate on the macroscopic effects, yet it is very hard to believe that the microscopic effect of erosion will not dominate over those minor considerations. At 0.3c the proton hitting the shield would only have to impart a few billionths of its kinetic energy on a surface molecule that it disturbs for that molecule to fly free. In theory, each proton could loosen millions of (metal) atoms, but it would more likely only effect just a handful, Still we are talking about the potential loss of about a 100-1000 kg of metal for every kg of ISM intercepted. I think current ideas of shielding around or above 0.1c may turn out to be fantasy.

It looks like you have used cm/s as the velocity when it should be m/s, I get approx ~600W/m^2 (~body temperature with an ideal radiator), I had fat fingers by putting a 2 in the mass of the proton in my first result.

The radiation output from the exhaust down the channel will be around 10-100 MW of spectrum radiation, if I remember 50 % at~10000 K is the ionisation temperature of hydrogen, gamma rays would interact with ionised hydrogen atom directly. That amount of radiation is going to hit something for sure. I can’t see the ISM been a problem, a rock will be a problem though!

Average space density is likely be 1 atom per cubic cc, but in the local interstellar cloud may be a tenth of this, or in the local bubble a twentieth (0.05 atoms/cc), which would adjust your calculated result in the range from 4 – 80 W/cm^2

What this suggests to me is that the erosion problem is going to be severe in the cool dense ISM, but negligible in the hot, ionized ISM (where repulsive fields may work). Therefore navigation is going to be important and that ISM density maps will be as important to stellar craft as winds and currents were to sailing ships.

Do we need to send out fast probes to do the mapping (slow, limited data, and expensive), or can we do this with telescopic instruments within the solar system?

‘Eniac, I note you continue to concentrate on the macroscopic effects, yet it is very hard to believe that the microscopic effect of erosion will not dominate over those minor considerations. At 0.3c the proton hitting the shield would only have to impart a few billionths of its kinetic energy on a surface molecule that it disturbs for that molecule to fly free.’

If we have 1 proton/cm^3 and we went all the way to A Centauri every one of the surface atoms would be hit-on average-. It is quite true that a hit would disrupt the chemical bonds of a large number of atoms but I see only a few would escape from the surface, lets say 1000. Now there are 6 x 10^-9 kg of material per cm^2 all the way to A.C so lets say we lose 1000 times that which is 6 x 10^-6 kg or about 60 grains of salt per cm^2

There are ways to reduce the loss as well, if we say made the shield from iron and a powerful magnetic field applied it could force some of the atoms to return to the shield by magnetism, I like the adjustable magnetic liquid best. We could also have a thin layer of low density material on top which would allow the atoms in but not allow the sputtered atoms back out or at least reduce the number of escapees.

Thanks ProjectStudio for the link

Space is empty. You just won’t believe how vastly, hugely, mind- bogglingly empty it is. I mean, you may think your refrigerator is empty, but that’s just an empty tin of peanuts to space.

Michael, I love simplicity. Rather than mix units (eg use of cm and joule/watt instead of erg) allow me to stick to SI. Your atom per cm3 becomes a million per cubic meter.

Now let us work with that ISM to alpha centauri (4×10^16m away). We thus intercept 4×10^22 atoms/sqm. If we lost a thousand W atoms (with a molecular weight of 3×10^-25kg) to each proton that would be a loss of
4×10^22 x 3×10^-25 x 1000 = 12kg/sqm.

This is not insuperable if it is kept to below this, but is significant. I think you forgot that shield atoms will likely be 25 – 300 times heavier than the proton (with B or C being the only notable exceptions)

One thing I must ask: is it possible to have waves in the ISM that are, say, 100x denser? If it is, then hitting a small patch of it would melt the shield at 0.3c anyway.

Michael is right, I used the wrong units for the proton energy, thereby being off by four oders of magnitude. Good thing, too! So, thick shields should work pretty well up to 0.6 or 0.7 of c. Sorry about the false alarm, I feel terrible.

Rob Henry has a good point about sputtering, one I have made myself before. I think it applies more to thin sails or ionization foils, less to thick shields where most of the time the particle travels in the interior where displaced atoms cannot escape. You will get crystal structure defects, but metals should do just fine with that, perhaps even improve. There will be ablation at the surface, though, and Michael has provided a reasonable estimate for that..

Alex is right about the great variety of ISM that is out there, except that I think it varies on a scale much larger than individual stars, such that we are probably stuck with a pretty even density of whatever it happens to be in our neighborhood. As ProjectStudio says, it seems we are lucky and it is quite a bit less dense than the average. So, all is good with thick shields, as long as we are not talking significant gamma factors (Sorry, Tau Zero…).

Thin sails, on the other hand, as well as micro-spacecraft, will probably still be quickly overwhelmed by the sputtering. This should be confirmed by actual calculations, of course. Particle physicists have a lot of experience firing energetic particles through thin foils. I believe I read somewhere those do not last long, but I am not knowledgable about that, myself.

Perhaps it would help if I give a ballpark estimate of the extent of the erosion, especially since I have never seen that done before. Here goes.

The protons hit the shield at such high velocities that we may expect that the first cause of friction will be more due to broken bonds than molecules vibrating within a lattice. Let us call the proportion of all energy that goes into breaking bonds X. If the shield is warm, those bonds will reform. If the shield is cold, they will not causing it to become brittle. Now a kilo of H at 0.3c has 4000 million MJ, thus breaking 900 million Xkg of tungsten.

Next we need to know what proportion of those molecules are near enough the surface to escape. Michael mentioned a 300 micron penetration depth, but the energy distribution will be skewed toward the surface. I will assume that skew conveniently cancels X, and that only the very top surface layer can escape. 300 microns is close to a million interatomic distances.

Finally we have it, about a ton of shield for every kg of ISM intercepted – so Michael’s earlier estimate might have been right to within an order of magnitude, and mine a couple of orders too high.. Mind you, doing the exercise shows how a tiny bit of ISM can make a huge amount of shield brittle if it ever gets to cold for broken bonds to reform!

Aren’t we likely to get that just by random distributions? Then there are the wave fronts due to nova/supernova explosions that I assume are higher densities than their local ISM. It just seems unlikely to me that the ISM is evenly distributed except on large scales. At some scales there is likely to be density variation that would affect the ship. If so, local density changes could feel like more than mild “buffeting”. I see this as analogous to the solar weather, but at different scales (although this scale may be greater than the distance to alpha Centauri.)

It appears that the yield values are generally not far from 1, never above 10. This bodes well, as it would mean that surface erosion could be quite minimal. Assuming a yield of 5, we might lose only 10 or so microns on a trip to Alpha Centauri. This is bad if you want a sail that is much thinner than that, but it should work very well for a thick solid shield. I am beginning to feel outright optimistic about this issue.

Oh, it helps noting that I assume the amount of ISM material between here and Alpha Centauri to amount to a layer of a few microns, were it condensed. I recall once calculating that to be the case, but we all know the reliability of my calculations by now….

‘Now let us work with that ISM to alpha centauri (4×10^16m away). We thus intercept 4×10^22 atoms/sqm. If we lost a thousand W atoms (with a molecular weight of 3×10^-25kg) to each proton that would be a loss of
4×10^22 x 3×10^-25 x 1000 = 12kg/sqm.’

Good spot Rob, i forgot it was heavier atoms that would be ejected but surprisingly there are only a few ejected at the surface as most of the energy is dumped in the interior of the material, think of the bragg peak.

Perhaps nanotubes aligned with the axis could do the job of channeling the oncoming material into the interior where there is no escape.

‘If the shield is warm, those bonds will reform. If the shield is cold, they will not causing it to become brittle.’

Hydrogen embrittlement is a possible issue but annealing could possibly be used to remove it. I am thinking of a possible magnetic liquid that can be adjusted in depth so that if a large dust particle is sensed it could be thickened in that area before impact.

@Eniac November 12, 2014 at 23:27

‘Good thing, too! So, thick shields should work pretty well up to 0.6 or 0.7 of c. Sorry about the false alarm, I feel terrible.’

Don’t worry about it, I once had the job of getting some expensive ceramic bearings made up for a machine, I asked for imperial when the machine was metric. The engineer who had the job of putting the imperial bearings on the metric shafts was found in a great deal of sweat and distress! I am sure they have all forgotten about it by now but I haven’t.

So in order to protect the craft we may need active, geometric and passive protection but I don’t see a great issue about it. Larger dust is an issue, one hit of a grain of sand and there is going to be a big bang.

Here’s a question : If we use a Ramscoop to decelerate , would there be a profitable element of ”recycling” of particles ? I can imagine that a certain fraction of the exhaust particles wil collude repeatedly with other particles ,gradually slowing them down and perhabs leaving a trail of awaiable material for the ramscoop to re-use ?

@ole Burde, if the ejected particles collide with the ISM, it will increase their velocity in the direction of flight. This will reduce the effectiveness of the ISM as a braking medium. So recycling in this case is just like recycling rocket exhausts, it cannot work. If we use the ramscoop idea, we really need to fire the particle exhaust as a narrow beam outside the cone of the scoop so that we get the thrust benefit without disturbing the scoop drag.

Allex Tolley : Firing outside the cone is the first-order logic solution , but perhabs there is another : What if multiple collutions will bring the fired particles to a low average speed far far awy from the vehicle ,most of the energy having been translated into radiation . The ramscoop would then meet a bigger concentration of particles MUCH later in the process , perhabs several decades later , and this might happen exactly when the speed otherwise would fall below the minimum value of effective ”scooping” …It would demand that the beam would be extreemely narrowly focussed in order to NOT desturb mot of the stuff being scooped (exept for very small spot in the middle) from the medium range path of the vehicle , but this problem of focussing a beam narrowly for an extreemly long range , is one that will have to be solved anyhow in order to get accelerated away from earth .

Thanks Michael for that Bragg peak information. It certainly helps limit the problem.

Thanks for that sputtering reference Eniac. The results seem at least an order of magnitude less than my estimate, and in line with what might be expected if factoring in that Bragg peak – interesting. A quick and simplified calculation by me puts the energy of a .3c proton at 50 MeV and these were 12 MeV.

On the completely different matter of the internal structure becoming brittle, though metal bonds are different than ceramic, I am pretty sure the bonds will not reform at cryogenic temperatures – though their activation energies will be lower. My guess is that 50-100 K is where that trouble might start, and I would love to see that experiment repeated at those temperatures.

Rob: Even though my most prohibitive calculations were, luckily, wrong; at 0.3 c we are still talking an energy deposition comparable to terrestrial sunshine on a black surface, which would make cryogenic temperatures very difficult to maintain. And why would we want to?

I was thinking along the lines of a ferro-fluid in which we can control a peak to face an on coming particle very fast. If we can do that we could handle much larger particle impacts. It would be a compromise between the mass of the system used to control it and its effectiveness at stopping large particles. My thought are of using a metal eutectic, maybe Galinstan, which has a low vapour pressure and melting point (-19 C) mixed with a nano-ferro magnetic material. There are lower ones than this.

A Eniac writes ‘cryogenic temperatures would be very difficult to maintain’. at the front of the craft due to on coming material, he is correct and this would benefit us. My worry at high velocities is neutron spalling and secondary radiation.

As for slowing down a magnetic sail I wonder if it is possible to have a close approach of a red dwarf (we can get very close) and enter the leading rotation region of the star and cut its powerful magnetic field. There are sure to be a lot of ions trapped in a belt around the equator which would interact with the crafts magnetic field as well adding to the braking effect of the magnetic field coupling.

@Michael, I’m not sure that a ferrofluid is what you want as it is a solid liquid mix – or colloid. Those galinstan type liquids sound more promising iff all their main constituent (surface) atoms are magnetic.

@Eniac, estimates of the ISM in our region vary. Above, someone thought that one of the best estimates for it locally was only a 20th our standard million/m3 working figure. In ceramic, even 300K is not warm enough to remove all the damage.http://en.wikipedia.org/wiki/Thermoluminescence_dating
At lower temperatures we will have even more problems. I estimate that about a 10% the energy of these particles showers will go into breaking bonds. From my above figure for a W shield, that would mean a single gram of ISM powderising to the atomic level 100 tons of tungsten. The grain size at failure should be well above atomic level, so perhaps the true figure is even worse.

Michael, If you are breaking close around a red dwarf after having come of 0.3c then you are a braver man than I. My thinking is, that for the extra paraphernalia and its optimization to be worth it the breaking effect would have to be significant. If we had already bled off 99.9% of our kinetic energy, we would still be traveling at 1% c. Now at such speeds we would pass through a path of 0.1AU in 5000 seconds, so breaking a sizable chunk of the rest of the kinetic energy with 0.1% of that energy going into vibrational heating would cause a payload/sail with the specific heat capacity of water to want to heat up at the rate 200K/s. Most materials have a lower specific heat capacity than water.

‘I’m not sure that a ferrofluid is what you want as it is a solid liquid mix – or colloid. Those galinstan type liquids sound more promising iff all their main constituent (surface) atoms are magnetic.’

It does not matter if the surface atoms are magnetic only that the liquid response to a controlling field to shift fast.

‘At lower temperatures we will have even more problems. I estimate that about a 10% the energy of these particles showers will go into breaking bonds. From my above figure for a W shield, that would mean a single gram of ISM powderising to the atomic level 100 tons of tungsten. The grain size at failure should be well above atomic level, so perhaps the true figure is even worse.’

The bonding strength of materials is meaningless at such high velocities, even if it is a problem perhaps we could inject a metallic liquid into the powered tungsten material to keep it intact with a much lower melting point.

‘Now at such speeds we would pass through a path of 0.1AU in 5000 seconds, so breaking a sizable chunk of the rest of the kinetic energy with 0.1% of that energy going into vibrational heating would cause a payload/sail with the specific heat capacity of water to want to heat up at the rate 200K/s.’

There is a lot of energy to get rid of, I think even vaporising hydrogen, which has a higher heat capacity than water would not remove the energy. Maybe using the technique below a certain velocity would be better.

Yes Michael, the bonding strength WOULD be meaningless if it is warm enough for bonds to reform or if we are in a true liquid. Eg,, at a guess, those colloidal particles in a ferrofluid might get broken up ever finer by the protons till they loose their magnetism.

‘Eg,, at a guess, those colloidal particles in a ferrofluid might get broken up ever finer by the protons till they loose their magnetism.’

Iron does not lose its magnetism at the atom scale.

Now as to whether tungsten would be a suitable shield material it appears not, the energy aborbed during the 4.3 ly trip to A.Cent would need a shield over three meters thick per cm^2 (7.7 kg) to prevent all the bonds been broken.

Hydrogen would have no such problem which would require only 0.061 kg per cm^2 or much less because of the rebonding process that would take place naturally. Keeping the hydrogen contained at temperature would be an issue though but not insurmountable and compounds of hydrogen could do the trick.

As for slowing down the sail I am still thinking of a fission reaction start up at the start of the deceleration phase which spreads the fission/products onto the target star side of the sail, in essence using a fission fragment process.

Why use photons if you’re not going near the speed of light but are only going at a maximum of 5-10% or 30% of c?

It’s 10-100x more efficient to send a beam of subrelativistic dust particles (initially charged for acceleration in a large, linear accelerator then neutralized to keep them from being pushed around by the solar wind and magnetic fields). The particles can be vaporized, perhaps by a laser or a particle beam on the target craft, as they near the target craft and then the subrelativistic plasma’s momentum can be captured by a small magnetosail (and some of its energy used to power the laser). Importantly, this also means you don’t need enormous laser optics and you could actually contemplate engineering this sort of thing.

Once you near the target system, you would expand the magnetosail to brake against first the interstellar medium and then the stellar wind of the target system.

I think this combination is much more efficient and practical than almost any other system of interstellar travel I’ve seen mentioned. And we could use it for exploring the outer portions of our solar system (like the Oort cloud), as well. The high efficiency of the concept (and lack of a need for solar-system-spanning optics precise to within tenths of microns) would allow it to possibly be built on some scale in this century, and certainly some of the main technical uncertainties (how do you accurately aim subrelativistic dust particles? how do you build an accelerator capable of gigawatts of efficient acceleration? how do you vaporize subrelativistic dust particles efficiently and precisely at a distance?) could be resolved at the lab scale this century.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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